相信很多小伙伴在看深刻理解Java虚拟机的时候,做者给咱们举例一个demo来发生PermGen spaceexpress
一、经过List不断添加String.intern();this
二、经过设置对应的-XX:PermSize与-XX:MaxPermSize(更快看到效果),spa
三、在jdk1.6的环境下会抛出OOM:PermGen space异常对象
public static void main(String[] args) { List<String> str=new ArrayList<>(); int i=0; while (true){ str.add(String.valueOf(i).intern()); } }
然而在jdk1.8的环境下,这段代码,会出现OOM,但不是出现PermGenSpace,而是会当堆内存不够用(-Xmx)的时候,抛出Java heap space,并且会舒适提示内存
ignoring option PermSize=10M; support was removed in 8.0ci
ignoring option MaxPermSize=10M; support was removed in 8.0rem
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首先咱们看一下String.intern()方法,官方的注释是虚拟机
Returns a canonical representation for the string object. A pool of strings, initially empty, is maintained privately by the class String. When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned. It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true. All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java? Language Specification.string
大概意思就是,若是常量池已经存在这个string对象,那么就返回这个字符串的引用。不然将此string对象加入常量池,再返回引用
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可是在jdk1.8中,其实已经没有永久代这一说了,取而代之的是一个叫元空间(Meta space)。而常量池放到了堆中,因此也就不会出现PermGen space了
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那么若是想看到metaspace的异常怎么作呢?
一个是能够把这两个值设置的足够小,那么启动就会报错了。
-XX:MetaspaceSize=3M -XX:MaxMetaspaceSize=3M
一个是能够使用jdk动态加载技术,例如cglib动态的生成大量的数据来达到